Hydrogen is a key building block of many chemical processes. Virtually all hydrogen produced to date is used for chemical processes. Hydrogen can be produced in several ways, but economically the most important processes involve producing hydrogen from hydrocarbons. Fossil fuels including coal, oil and natural gas currently are the main sources of hydrocarbons for hydrogen production and biomass is an emerging source used, for example, in the production of biofuels. Gas streams containing hydrogen production as product or by-product of the processing of all of these hydrocarbon sources often contain corrosive gases such as hydrogen sulfide (H2S) and halides that attack metals on contact.
Of all the trace corrosive gases in the gas stream, H2S is one of the most problematic for metal and metal alloys. Various methods have been developed to protect metal components of gas purification devices or membranes from these corrosive gases, such as alloying or incorporation of polymeric coatings. Efforts in alloying for sulfur resistance have only resulted in marginal improvement due to the basic fact that metals are prone to be attacked by highly corrosive gases, especially in high temperature environments (e.g., temperatures above 300° C.) typically found in process gas streams from which hydrogen is to be recovered. None of these methods selectively allow hydrogen to pass through and at the same time protect metal membranes from corrosive gases, such as H2S, in elevated temperature environments.
In hydrogen purification systems employing metal membranes or metal sheet-like layers, hydrogen molecules travel to and contact a first surface of a metal membrane, e.g., palladium (Pd) membrane, and are split into hydrogen atoms by the catalytic reaction or effect at catalytically active sites on the palladium membrane. The hydrogen atoms then transport through the palladium membrane or metal layer and recombine at the second or opposite surface of the membrane as hydrogen molecules. Corrosive gases, such as the sulfur based gases, can react with the catalytically active sites of the metal membrane reducing the number of available sites and the separation performance of the membrane. In order to minimize or avoid such reactions with corrosive gases, they must be removed prior to process gases being separated through the membrane. Scrubbing or polishing steps used for such removal can add extra costs to commercial operations and can still leave enough residual amounts as to be problematic. For example, as reported in U.S. Pat. No. 3,350,845 to McKinley, even with a reduction in sulfur content to as low as 4 ppm, such small amounts are still sufficient to reduce hydrogen permeance across a palladium foil by 70% at 350° C. Reduction of catalytically active sites can also be problematic in hydrogen sensor applications that use palladium or other similarly affected metals or metal alloys in the sensing element.
In view of the foregoing, it would be desirable to provide a coating that protects contaminant-sensitive, metal-containing membranes or sensors, such as those formed of palladium or palladium alloy. It would also be desirable to provide a coating that separates out contaminant gases and allows high hydrogen permeation rates or flux, particularly in a high temperature corrosive environment.